A related-art image forming apparatus, such as a laser printer, generally includes a plurality of development units that accommodate different colors of toner; a plurality of photosensitive drums that are supplied with the toner from the plurality of development units though development rollers, respectively; a belt disposed opposite the plurality of photosensitive drums; and a plurality of transfer units that transfer the toner on the plurality of photosensitive drums toward the belt, respectively. In this related-art image forming apparatus, when a sheet conveyed by the belt passes between each of the photosensitive drums and the corresponding transfer unit, a transfer bias corresponding to the charging polarity of toner is applied to corresponding transfer units, whereby the different colors of toner held on the respective photosensitive drums are separated by the respective transfer units. Accordingly, the toner is sequentially transferred onto a sheet so that color printing of the sheet is performed.
In order to reduce or prevent toner, which has not been separated by a transfer unit, from being left on a photosensitive drum, a related-art image forming apparatus removes electricity from the toner still remaining on the photosensitive drum after transfer operation to thus disperse the toner over the photosensitive drums. Herein, the toner, which is remaining on the photosensitive drum after transfer operation, is also referred to as “residual transfer toner”. Then, the residual transfer toner is collected by the development rollers (see JP-A-5-53482). In another related-art, collecting the residual transfer toner is performed by changing a voltage applied to cleaning rollers slidably contacting with the photosensitive drums, respectively so that the cleaning rollers temporarily hold the residual transfer toner. Then, at a specific timing, the residual transfer toner is moved from the cleaning rollers to the photosensitive drums, thereby collecting the residual transfer toner by the development rollers.
Incidentally, in the above-described related-art image forming apparatus, if paper dust adhering to a sheet is moved to the photosensitive drum at the time of transfer of toner to the sheet, transfer operation performed in subsequent processes is sometimes disturbed by the paper dust, which obstructs appropriate printing operation. For this reason, in the related-art image forming apparatus, it is necessary to provide a mechanism for collecting paper dust. However, in the related-art art, if the mechanism for collecting paper dust is provided for each of the development unit in addition to the cleaning roller, the size of overall image forming apparatus increases.